1. Field of the Invention
The present invention relates to an improvement on the method of removing, through chemical etching, a damaged layer that is generated on the surface of a monocrystalline silicon wafer during a process of producing the wafer.
2. Description of the Related Art
A conventional process of producing a mirror-polished semiconductor wafer typically comprises the steps of slicing a monocrystalline ingot of silicon or the like to obtain a semiconductor wafer; and chamfering, lapping, acid etching, mirror-polishing, and cleaning the sliced semiconductor wafer. Depending on the required specifications, the sequence of steps is changed; some steps are repeated a plurality of times; or other steps such as heat treatment and grinding are added to or replace the above-described steps. Thus, a variety of kinds of steps are performed in accordance with the specifications.
Among the above-described steps, acid etching is performed for the purpose of removing a surface damaged layer introduced in the course of mechanical machining steps such as slicing, chamfering, and lapping. In the acid etching step, the surface of a wafer- is etched to a depth of a few to a few tens of microns through use of mixed acid aqueous solution composed of hydrofluoric acid, nitric acid, acetic acid, and water. However, acid etching involves the following problems:
1) The flatness of a wafer after lappingxe2x80x94which is indicated by thickness variation represented by, for example, TTV (Total Thickness Variation) (xcexcm) or LTVmax (Local Thickness Variation) (xcexcm) xe2x80x94deteriorates as the etching amount increases.
2) A waviness of a millimeter order or an uneven region called xe2x80x9cpeelxe2x80x9d is generated on an etched surface.
3) Harmful NOx is generated due to etching. In consideration of these problems, alkali etching is used in some cases.
Alkali etching has the following advantages:
a) Flatness established by lapping is maintained after etching.
b) Generation of harmful gas is suppressed. However, alkali etching has the following disadvantages:
i) If foreign mater enters pits locally existing on an etched surface and having a depth of a few microns and a diameter of a few to twelve or thirteen microns, the foreign matter causes generation of particles and/or contamination in a subsequent step.
ii) Since deep pits exist and surface roughness (Ra) increases, a polishing stock removal in a subsequent step of mirror-polishing (mechano-chemical polishing) must be increased.
iii) Since an etched surface has a sharp uneven shape compared to a surface etched through acid etching, the unevenness itself serves as a source of particles.
Accordingly, particles generated in a subsequent step and a polishing stock removal in a mirror-polishing step can be decreased if etching treatment can be performed while flatness attained through lapping is maintained, so as to remove a mechanically formed damage layer, improve the surface roughness, efficiently decrease the depth of deep pits locally formed due to the etching, and smooth the uneven shape of the surface.
The present invention has been accomplished to solve the above-mentioned problems, and an object of the invention is to provide a method of processing a semiconductor wafer which can remove a mechanically formed damage layer, improve surface roughness, and efficiently decrease the depth of locally formed deep pits, while the flatness of the wafer attained through lapping is maintained, in order to produce a chemically etched wafer (CW) having a smooth and flat etched surface that hardly causes generation of particles and contamination.
Another object of the invention is to provide a semiconductor wafer processed through the above-described processing method.
To achieve the above object, the present invention provides a method of processing a semiconductor wafer sliced from a monocrystalline ingot. The method comprises at least the steps of chamfering, lapping, etching, mirror-polishing, and cleaning and is characterized in that in the etching step alkali etching is first performed and then acid etching is performed, and that an etching amount of the alkali etching is greater than an etching amount of the acid etching.
In the etching step of the processing method of the present invention, after the step of lapping alkali etching is first performed in order to remove a mechanically formed damage layer, while the flatness of the wafer attained through lapping is maintained, and subsequently, acid etching is performed in order to decrease the depth of locally formed deep pits remaining after the alkali etching and to improve the surface roughness and the sharp uneven shape.
At this time, the etching amount of the alkali etching must be set greater than the etching amount of the acid etching because of the following reasons. That is, in order to decrease the depth of locally formed deep pits remaining after the alkali etching, the etching amount of the alkali etching must be increased to a certain level, which is greater than the etching amount of the acid etching required for decreasing the rate of generation of faults such as stain stemming from unevenness in etching and for improving flatness.
Preferably, before being subjected to the acid etching a wafer that has undergone the alkali etching is immersed into aqueous solution of hydrogen peroxide.
The surface of a wafer that has undergone the alkali etching is active and hydrophobic, so that foreign matter easily adheres and dirties the wafer. However, if the surface of the wafer is oxidized through immersion into aqueous solution of hydrogen peroxide and thus made hydrophilic, particles hardly adhere to the wafer surface.
Preferably, the etching amount of the alkali etching is 10-30 xcexcm, and the etching amount of the acid etching is 5-20 xcexcm
In the alkali etching, there is a tendency that the depth of locally formed deep pits remaining after the alkali etching decreases with increasing etching amount, and that the surface roughness becomes higher with increasing etching amount of the alkali etching. Therefore, the etching amount of the alkali etching is maintained within the above-described range. In the acid etching, as the etching amount increases, the stain generation rate decreases considerably although the flatness deteriorates. Therefore, the etching amount of the acid etching is maintained within the above-described range.
Preferably, the etchant used in the alkali etching is an aqueous solution of NaOH or KOH, and the etchant used in the acid etching is a mixed acid aqueous solution composed of hydrofluoric acid, nitric acid, acetic acid, and water.
When such etchants are-used, etching is performed effectively and reliably in both the alkali etching and the acid etching, and the respective etching amounts can be controlled with relative ease. In addition, the etching can be performed at low cost.
In the present specification, each specific value used in relation to etching amount represents the sum of the thicknesses of layers removed, through etching, from opposites surfaces of a wafer.
Preferably, the acid etching is reaction-controlled acid etching.
When the acid etching is of a reaction-controlled type, the flatness can be further improved through suppression of waviness, while realizing a decrease in the depth of deep pits locally remaining after the alkali etching and improvement of the surface roughness and the sharp uneven shape.
Preferably, in the reaction-controlled acid etching, there is used an etchant obtained through addition of 20-30 g/l of silicon into a mixed acid aqueous solution composed of hydrofluoric acid, nitric acid, acetic acid, and water.
When such etchant is used, etching is performed effectively and reliably, and the etching amount can be controlled with relative ease. In addition, the etching can be performed at low cost.
The present invention provides another method of processing a semiconductor wafer sliced from a monocrystalline ingot. The method comprises at least the steps of chamfering, lapping, etching, mirror-polishing, and cleaning and is characterized in that in the etching step reaction-controlled acid etching is first performed and then diffusion-controlled acid etching is performed, and that an etching amount of the reaction-controlled acid etching is greater than an etching amount of the diffusion-controlled acid etching.
In the etching step of the processing method of the present invention, reaction-controlled acid etching is first performed for a lapped wafer in order to remove a mechanically formed damage layer, while the flatness of the wafer attained through lapping is maintained, and subsequently, diffusion-controlled acid etching is performed in order to decrease the depth of deep pits remaining after the reaction-controlled acid etching and to improve the surface roughness and the sharp uneven shape.
At this time, the etching amount of the reaction-controlled acid etching must be set greater than the etching amount of the diffusion-controlled acid etching because of the following reasons. That is, in order to decrease the depth of locally formed deep pits remaining after the reaction-controlled acid etching, the etching amount of the reaction-controlled acid etching must be increased to a certain level, which is greater than the etching amount of the diffusion-controlled acid etching required for decreasing the rate of generation of faults such as stain stemming from unevenness in etching and for improving flatness.
Preferably, the etching amount of the reaction-controlled acid etching is 10-30 xcexcm, and the etching amount of the diffusion-controlled acid etching is 5-20 xcexcm.
In the reaction-controlled acid etching, there is a tendency that the depth of locally formed deep pits remaining after the etching decreases with increasing etching amount, and that the surface roughness increases with increasing etching amount of the reaction-controlled acid etching. Therefore, the etching amount of the reaction-controlled acid etching is maintained within the above-described range. In the diffusion-controlled acid etching, as the etching amount increases, the stain generation rate decreases considerably although flatness deteriorates. Therefore, the etching amount of the diffusion-controlled acid etching is maintained within the above-described range.
Preferably, in each of the reaction-controlled acid etching and the diffusion-controlled acid etching, there is used an etchant obtained through addition of silicon to a mixed acid aqueous solution composed of hydrofluoric acid, nitric acid, acetic acid, and water, and the silicon concentration of the etchant used in the reaction-controlled acid etching is higher than that of the etchant used in the diffusion-controlled acid etching.
When such etchants are used, etching is performed effectively and reliably in both the reaction-controlled acid etching and the diffusion-controlled acid etching, and the respective etching amounts can be controlled with relative ease. In addition, the etching can be performed at low cost.
Preferably, the silicon concentration of the etchant used in the reaction-controlled acid etching is 20-30 g/l, and the silicon concentration of the etchant used in the diffusion-controlled acid etching is 5-15 g/l.
When the silicon concentration of the etchant used in the reaction-controlled acid etching is less than 20 g/l, the etchant becomes a diffusion-controlled-type acid, so that flatness deteriorates. When the silicon concentration of the etchant used in the reaction-controlled acid etching exceeds 30 g/l, the etching rate decreases, and a longer period of time is required for dissolving silicon in a mixed acid aqueous solution in order to prepare the etchant. Therefore, the silicon concentration of the etchant used in the reaction-controlled acid etching is preferably adjusted to fall within the range of 20 to 30 g/l. When such reaction-controlled type acid etchants are used, etching is performed effectively and reliably, and the etching amount can be controlled with relative ease. In addition, the etching can be performed at low cost.
In the diffusion-controlled acid as well, small amount of silicon is preferably dissolved into the mixed acid aqueous solution in order to prevent variations in the etching rate, which would otherwise be caused by variations in the composition of the solution. When the silicon concentration is less than 5 g/l, a variation in the composition of the solution causes a large variation in the etching rate. When the silicon concentration is exceeds 15 g/l, the etching rate decreases, and the state of the surface of a wafer after etching becomes similar to that obtained through etching by use of a reaction-controlled-type acid, so that the surface roughness increases. Therefore, the silicon concentration of the etchant used in the diffusion-controlled acid etching is preferably adjusted to fall within the range of 5 to 15 g/l.
The present invention further provides a semiconductor wafer processed by either one of the above-described methods of the present invention. As described above, in one method of the present invention, alkali etching is first performed in order to remove a mechanically formed damage layer, while the flatness of the wafer attained through lapping is maintained, and subsequently, acid etching is performed. Therefore, there can be obtained a semiconductor wafer in which the depth of deep pits remaining after the alkali etching is decreased and the surface roughness and the sharp uneven shape are improved. Especially, when reaction-controlled acid etching is employed as the acid etching, the degree of waviness decreases, so that a semiconductor wafer having a flatter surface can be produced.
The above-described wafer can be obtained through the other method of the present invention, in which reaction-controlled acid etching is first performed and then diffusion-controlled acid etching is performed and in which the etching amount of the re-action-controlled acid etching is greater than the etching amount of the diffusion-controlled acid etching.
The present invention further provides a semiconductor wafer in which an LTVmax measured in cells of 20xc3x9720 mm is 0.3 xcexcm or less, and the maximal value of pit depth is 6 xcexcm or less. In this case, the average value of waviness of the semiconductor wafer is preferably 0.04 xcexcm or less.
As described above, according to the present invention, the flatness of the wafer attained through lapping is maintained; the degree of waviness of the wafer surface after etching is decreased; deep pits are prevented from being locally generated; and degradation of surface roughness is suppressed. Thus, there is obtained a chemically etched wafer having a smooth and flat etched surface that hardly causes generation of particles and contamination such as stain. Therefore, the amount of stock removal in a mirror-polishing step can be decreased, and the flatness of the wafer can be improved.